The present invention relates to an optical switching matrix that can be used in particular in an exchange or node of an optical fiber data-transmission network. The matrix can also be used in any other routing member of such a network.
Such a matrix includes inputs and outputs, each input receiving data to transmitted, and each output transmitting, from the matrix, that data which is received via the input to which the output is temporarily connected. Switching signals are received by the matrix so as to control the beginning and the end of the period during which the output is connected to the input. Inside the matrix, data to be transmitted is carried by light waves.
An important characteristic of such a matrix is its ability to perform data collection operations easily when necessary, so as to collect a plurality of different items of data which are received via a plurality of respective inputs, and which are all to be transmitted to the same output.
A first known optical switching matrix includes the following elements (see FIG. 1):
a number Z of light emitters such as the emitter 100i, the entire set of emitters being designated by the reference 100; each of the emitters has a wavelength at which it emits a carrier light wave; the wave is modulated by data to be transmitted that is received by the emitter, e.g. in electrical form; the emitters constitute tuned members that are tuned to wavelengths; PA1 a wave distributor 102; such a distributor is provided both with Z inputs respectively connected to Z emitters 100 and also with Z outputs, and it is referred to below as a "star distributor"; it permanently transmits a fraction of a mixture constituted by all the waves that it receives at its inputs to each of its outputs; such a distributor is often referred to in the literature as a "star coupler"; and PA1 Z inputs 200 receiving data to be transmitted, the data being received in optical form; PA1 Z outputs 204 transmitting the data in optical form; and PA1 a switching distributor 202 variably connecting the inputs to the outputs; to make the connections, the distributor receives switching signals which control the guided paths formed in the distributor between the inputs and the outputs; the paths are guided in the same way for waves that may have different wavelengths; such a distributor is well known and itself constitutes an optical switching matrix such as the matrix described in a document by Hermes entitled "Optical Routing--State of the art and future aspects", Thomas Hermes, Proceedings ECOC'86, pp 33-38; some such distributors enable data to be broadcast from one input to a plurality of outputs, while others enable data received via a plurality of inputs to be collected at one output. PA1 members tuned to optical wavelengths of the matrix, the members firstly forming a set of emitters receiving data to be transmitted and having tuning wavelengths at which they emit waves carrying the data, the members secondly forming a set of filters also having tuning wavelengths, each filter selectively transmitting the waves whose wavelengths are equal to its tuning wavelength; and PA1 a distributor having inputs and outputs for distributing the waves received at its inputs to its outputs, the distributor being connected between the set of emitters and the set of filters; PA1 the matrix being characterized by the fact that it includes a plurality of said distributors firstly forming a group of switching distributors, each of which is capable of connecting each of its inputs to each of its outputs on command, the switching distributor inputs being connected to said emitters, and secondly forming a group of star distributors, each of which permanently connects all of its inputs to all of its outputs, the star distributor inputs being connected to the switching distributor outputs, and the star distributor outputs being connected to said filters.
Z filters such as 104j, given the overall reference 104 and respectively connected to the Z outputs of the distributor 102; such a filter has a wavelength and constitutes a tuned member that is tuned to that wavelength; it selectively transmits those waves whose carrier wavelengths are equal to its own wavelength; the filters such as 104 are referred to below as "fixed" filters to indicate that their wavelengths are fixed.
The emitters are referred to below as "controlled" emitters to indicate that their wavelengths are controlled as a function of the switching signals that are received by the matrix, so that when data to be transmitted is received by an emitter 100i and when that data is to be transmitted in optical form to an output of a filter 104j whose wavelength is Lj, the emitter is tuned to that wavelength Lj, i.e. the wavelength of the emitter is made equal to Lj.
That first known matrix uses a technique referred to internationally as "wavelength routing". This technique is described in a document by Brackett entitled "Dense Wavelength Divisions Multiplexing Networks: Principles and Applications", Charles A Brackett, IEEE Journal on Selected Areas in Communications, Vol. 8, No. 6, August 1990 p 948-964.
That first known matrix may be referred to as a "wavelength-division" matrix because it uses different wavelengths of the optical spectrum.
A second known matrix is shown in FIG. 2, and may be referred to as a "space-division" matrix because it creates guided light paths that can be varied in three-dimensional space. For example, that matrix includes:
Those known optical switching matrices suffer, in particular, from the drawback that their capacities are limited to values that are lower than those desired, their capacities being expressed in terms of numbers of inputs and of outputs. The capacity of a wavelength-division matrix is limited by the number of available carrier wavelengths. The available wavelengths are those lying in a wavelength band whose width is limited by various factors, such as the feasibility and cost of making the components, and the compatibility thereof with other members in the network. Said wavelengths must also be sufficiently far apart to avoid any cross-talk. The capacity of a space-division matrix is limited by the amount of space required for the guided paths which must be pre-established in the switching distributor. However, higher-capacity matrices can be constructed by associating limited-capacity matrix elements. But such associations require three successive stages of such matrix elements, so that any set of connections from among all the possible connections can be made simultaneously. This appears in the above-mentioned document by Hermes, and in a document by Clos entitled "A Study of Non-Blocking Switching Networks" Charles Clos, Bell System Technical Journal, Vol. 32, pp 406-424 (1953).